Recently, there is a trend to use a semiconductor laser for a light source of an image display device. Conventionally as these light sources, extra high pressure mercury lamps, LEDs and the like have been used. However, improvement of color reproductivity and expansion of color representation range are always required. The laser light obtained by induced emission has a very narrow band emission spectrum, whereby it has an excellent ability to represent a pure color. In addition, when 3 primary colors (red, green and blue) by the laser are combined, it is excellent in terms of the capability of representation in a very high color range compared with conventional light sources. Therefore, it is an attractive light source for image display devices. And at present, the problems of the size of visible light laser generator, power consumption, cost and the like which were obstacles to practical use have been overcome, and in practice, large-size rear-projection televisions capable of representing in a wide color range where a semiconductor laser is a light source are produced.
It is known that when a combination of 3 kinds of laser light (for example, blue: 460 nm, green: 532 nm, red: 635 nm) corresponding to the above-mentioned 3 primary colors is used, a range of 150 to 170% of the NTSC color range can be secured and pictures closer to the real thing than those on any other existing type of display can be obtained. For example, the color range which CRT displays can represent using a cathode ray tube is 60 to 70% of the NTSC color range and superior compared with the liquid crystal display, but it is considerably narrow compared with the color range which the combination of the above-mentioned 3 kinds of laser light can represent.
In this regard, the above-mentioned NTSC color range is based on the standard for the color range of analog television, designed by National Television System Committee (NTSC) in the U.S.A.
As mentioned above, the combination of the 3 primary colors of laser light is excellent in representing the color range but poses the problem of generating speckle noise when applied as it is. Specifically, the light emitted from a laser light source is a coherent light which is very susceptible to interference, whereby its application to image displays as it is results in constant occurrence of light ununiformity called speckle noise which is a flickering of the bright part and the dark part all over the image finally projected. Therefore, when the laser light is used as it is, the visibility ends up being worse compared with image display devices using a light source conventionally used. In order to improve that and to use a semiconductor laser as a light source for image display devices, it is essential to prevent adverse effects on visibility by reducing or removing speckle noise so as to make its light more incoherent. In this regard, the projected light without speckle noise is called incoherent light.
Various solutions to this problem have been proposed. For example, Patent Literature 1 discloses a method of moving a speckle pattern of the image projected on a screen by passing the laser light through a rotating diffusion element (for example, ground glass). When the rotating speed of the diffusion element is high enough, the eyes of viewers cannot detect the speckle and the speckle seems to disappear. However, it is necessary to mount a mechanism of rotating the diffusion element on an image display device, leading to a disadvantage of complication for the image display device.
Patent Literature 2 describes a device of the type which displays images by scanning on the screen using a laser light. In said device, by using a lenticular lens where cylindrical lenses are periodically arrayed in the scanning direction on the screen, a light incident angle on the focus of the lens is changed in accordance with the advancement of scanning laser light and a speckle pattern of the light projected on the screen is also changed, resulting in reduction of speckle noise. This device has an advantage of not using a driving mechanism but inevitably needs a special screen for projection. In addition, it has many limits such that it can be used only for scanning-type devices.
Patent Literature 3 and Patent Literature 4 disclose a technique to produce a fiber bundle element leading to reduction of speckle noise of laser light. It is characterized by using a fiber bundle consisting of many optical fibers, where the difference in length of any two optional fibers is larger than the coherence length of a light source. A larger number of optical fibers leads to more reduction of speckle noise. Therefore, in order to get a fiber bundle with good performance, dozens to hundreds of optical fibers different in length are required. This is thought to lead to a high manufacturing cost. In addition, a relatively large volume seems to be required in order to incorporate said bundle into a device and this cannot be said to be advantageous for incorporating into an image display device using a conventional laser light source.
Patent Literature 5 relates to a laser lighting optical device and discloses a method for reducing speckle noise by split-reflecting a light flux in its optical system using a plurality of stepped reflecting mirrors with optical path difference. In this method, spatial coherence is reduced by making the optical path length of the split light flux longer than the coherent length. As a result, a light flux with low interference is synthesized and speckle noise is reduced. In this method, a more split-number of the stepped reflecting mirror leads to a larger reduction effect on speckle noise. Therefore, it is considered that as the reduction effect is increased, a larger volume of the part removing speckles is required, and the same problem as in the case of the fiber bundle in Patent Literature 3 and Patent Literature 4 seems to be posed.
Patent Literature 6 discloses a method for reducing speckle noise, using a device comprising both a diffusion element into which a fluid component consisting of a fine particle dispersoid and a translucent dispersion medium are sealed and a fine particle oscillation-applying means by micro oscillation of the dispersoid (fine particle) by the oscillation-applying means when coherent laser light is passed through the element. In this method, said micro oscillation is forcibly performed by change in the alternating electric field, change in the magnetic field, or the ultrasonic wave (and so on). The speckle pattern of the diffused laser light diffused by said micro oscillation of said fine particles randomly changes at a high speed. People cannot detect the speckle pattern randomly changing at a high speed and speckle noise is thus removed. This method has, compared with the method requiring a mechanically driving part in Patent Literature 1 and the like, a possibility to make it smaller. However, said Patent Literature 6 provides no specific, technical disclosure relating to the average particle size of fine particles, the strength of electric field, magnetic field or ultrasonic wave, concentration of the fine particle in a dispersion and the like, nor practical disclosure about the way of controlling these and the degree of removing speckle, resulting in disclosure of nothing more than an idea.